1. Field of the Invention
The present invention relates to a light emitting apparatus used as backlight source for liquid crystal, illumination apparatus, light source for computer display, flash light for camera, ancillary light source for movie picture or the like.
2. Description of the Related Art
Cold cathode ray fluorescent tubes (CCFL) are currently used as the backlight source for the liquid crystal display of apparatuses other than mobile apparatus. However, it is strongly called for to replace the CCFL with light sources that do not use mercury, in order to avoid adverse effect on the environment. A light emitting apparatus that uses a light emitting device (hereinafter referred to as LED, too) receives much attention for that purpose.
A light emitting apparatus using a light emitting device is compact, has high power efficiency and emits light with clear color. The light emitting device is a semiconductor device and therefore never blows out. Moreover, the light emitting device has excellent initial drive characteristic and is highly durable against vibration and repetitive ON/OFF switching operations. Due to these excellent characteristics, the light emitting apparatus using the light emitting device such as light emitting diode or laser diode (hereinafter referred to also as LD) is used as the light source for various applications.
A white light emitting apparatus using LED may be constituted from any of the following three combinations.
First is a light emitting apparatus that combines a blue LED and a so-called YAG fluorescent material that emits yellow light. The light emitting apparatus emits white light by blending blue light and yellow light emitted by the YAG fluorescent material that is excited by the light emitted by the blue LED. This light emitting apparatus is widely used due to its low power consumption, capability to easily control the drive of the LED and capability of satisfactory color blending.
Second is a light emitting apparatus that combines an ultraviolet LED and fluorescent materials. Fluorescent materials that emit blue light, green light and red light are used. The light emitting apparatus produces white light through blending of blue light, green light and red light that are emitted by the fluorescent materials. Since the emission from the ultraviolet LED is hardly visible to the eye, color of the light emitted by the apparatus is determined solely by the light emitted by the fluorescent materials. This light emitting apparatus is capable of easily controlling the drive of the LED and satisfactory color blending.
Third is a light emitting apparatus that combines a blue LED, a green LED and a red LED. This light emitting apparatus is a so-called three-wavelength light emitting apparatus, and emits white light by means of light emitted by the LEDs. This light emitting apparatus has low power consumption and a wide range of colors displayed after passing a liquid crystal.
However, the first light emitting apparatus emits light that is generated by blending blue light and yellow light, and has less red component than the light emitted by the cold cathode ray fluorescent tubes (CCFL).
The second light emitting apparatus uses the ultraviolet LED, and therefore requires measures to prevent ultraviolet ray from leaking to the outside. Since the emission from the ultraviolet LED is hardly visible to the eye, ultraviolet ray that has leaked cannot be used as light and results in low light emitting efficiency.
The third light emitting apparatus is not capable of blending light satisfactorily, and has low color rendering properties. Since the light emitted by the LED is sharp unlike the light emitted by the fluorescent material, the light emitted by the LED is difficult to blend with each other. Also because one light source requires at least three LEDs, it is difficult to control the driving of the LEDs and adjust the color tone.